A single transistor, when used as an amplifier, may typically have a relatively low current gain and input impedance, especially as the signal frequency increases. One of the earliest transistor circuits to correct this problem was described in U.S. Pat. No. 2,663,806 to Darlington, whose disclosure is incorporated herein by reference. The disclosure shows a number of different configurations of pairs of transistors for increasing the gain and input impedance of transistor circuits, some of which became known as “Darlington Pairs.”
As transistors in an integrated circuit (IC) are reduced in size, further limitations such as reduced maximum allowable voltage drop place increased constraints on the performance of the IC. For an IC operating as an amplifier, the constraints, especially for high frequency amplifiers, lead to low output signal amplitude per amplifier stage.
Transistor cascode circuits, in which two or more transistors are connected in a series configuration, i.e., with a collector/drain of a first transistor connected to an emitter/source of a second transistor, are well known for being able to overcome some of the limitations described above. For example, U.S. Pat. No. 4,319,198 to Sosin, whose disclosure is incorporated herein by reference, describes a cascode-type amplifier in which the bases of the transistors are connected by a potential divider consisting of reactive components.
U.S. Pat. No. 5,032,799 to Milberger et al., whose disclosure is incorporated herein by reference, describes a radio frequency (RF) amplifier comprising a control amplifier connected to one or more “clone” amplifiers, all the amplifiers being connected in a cascode configuration. Each clone amplifier is configured to have unity gain, and biasing to the bases of the amplifiers is provided by one or more resistor chains.
U.S. Pat. No. 5,945,879 to Rodwell et al., whose disclosure is incorporated herein by reference, describes a cascode power amplifier, each stage of which has local negative feedback. The stages are coupled together with capacitors, obviating DC level restrictions for the individual stages.
U.S. Pat. No. 6,137,367 to Ezzedine et al., whose disclosure is incorporated herein by reference, describes DC and AC coupled multi-level cascode power amplifiers. In all the amplifiers transistor gates receive feedback from the drain of an output transistor in the cascode, via a resistor-capacitor network. The network sets levels of DC and AC signals applied to the gates.
U.S. Pat. No. 6,529,075 to Bruck et al., whose disclosure is incorporated herein by reference, describes a differential amplifier including cascode circuitry in two branches of the amplifier to cancel second and third harmonics of the output signal.
U.S. Pat. No. 6,366,172 to Hayashi et al., whose disclosure is incorporated herein by reference, describes a cascode amplifier in which the negative characteristic of an output conductance improves the conductance of the cascode.
In an article by Fraysse et al., entitled “A 2W, High Efficiency, 2–8 GHz, Cascode HBT MMIC Power Distributed Amplifier,” in 2000 IEEE MTT-S Digest, Volume 1, June 2000, Page(s): 529–532, and whose disclosure is incorporated herein by reference, a special circuit topology to increase output signal swing for wideband power distributed amplifiers, based on cascode cells, is described.
An article by Sackinger et al., entitled “A High-Swing, High-Impedance MOS Cascode Circuit,” IEEE Journal of Solid-State Circuit, 25(1), February 1990, whose disclosure is incorporated herein by reference, describes a cascode circuit in which the gate voltage of one of the cascode transistors is controlled by a feedback amplifier to increase output impedance.
An article by Deibele et al., entitled “Attenuation Compensation in Distributed Amplifier Design,” IEEE Trans. on MTT, 37(9), September 1989, whose disclosure is incorporated herein by reference, evaluates the use of negative resistance and capacitance of FET-based cascode circuits to increase gain-bandwidth product in distributed amplifiers, and compares the performance of these amplifiers with distributed amplifiers that do not include cascodes.
Notwithstanding the circuits of the prior art described above, limitations of cascode circuitry, including limitations on the current gain, the input impedance, the bandwidth, and the allowable signal swing, still exist.